Description
Packet-switched Core Network Node (PCN) is a collective term defined within 3GPP specifications, particularly in the context of policy, charging, and management frameworks. It refers to any node in the packet-switched domain of the core network that handles user data traffic and is subject to policy control and charging rules. The PCN concept abstracts the specific node types across different 3GPP system generations (GPRS, EPS, 5GS) into a common category for the purposes of policy and charging control (PCC) and network management.
Architecturally, PCN nodes are the workhorses of the data plane. In 2G/3G GPRS networks, the PCN includes the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). The SGSN is responsible for session management, mobility management, and routing data packets to/from the Radio Access Network (RAN). The GGSN acts as the anchor point to external packet data networks (e.g., the internet). In the 4G Evolved Packet System (EPS), the PCN encompasses the Serving Gateway (S-GW) and the Packet Data Network Gateway (P-GW). The S-GW is the local mobility anchor, while the P-GW provides the interface to external networks and performs deep packet inspection, policy enforcement, and charging. The Traffic Detection Function (TDF), an optional node, is also considered a PCN; it performs application detection and reporting, feeding into the PCRF for policy decisions.
How it works: User data packets flow through one or more PCN nodes. Each PCN node applies policies dictated by the Policy and Charging Rules Function (PCRF) via the Gx interface (for P-GW/GGSN) or other reference points. These policies govern quality of service (QoS), traffic shaping, and charging actions. For online charging, the PCN interacts with the Online Charging System (OCS) via the Gy interface. For offline charging, it generates Charging Data Records (CDRs) and sends them to the Charging Data Function (CDF) via the Ga interface. The PCN is thus a policy enforcement point (PEP) and a charging trigger function (CTF). Its role is to execute the decisions made by the control-plane policy entities.
Key components within a PCN node include the user plane processing functions (packet forwarding, tunneling using GTP-U), the control plane functions (GTP-C for session management), and the integrated policy and charging enforcement function (PCEF). The PCEF is a critical component that inspects packets, applies QoS marking (e.g., setting DSCP values), gates traffic flows, and meters usage for charging. The PCN's significance is that it provides a consistent point for applying business logic (policies) and monetization (charging) across evolving network architectures, regardless of the specific node name or protocol details.
Purpose & Motivation
The term PCN was introduced to create a unified, technology-agnostic reference for nodes involved in packet-switched data delivery within 3GPP standards. As networks evolved from GPRS to EPS and 5GS, the specific node names and some functions changed (SGSN/GGSN to S-GW/P-GW). However, the fundamental need to apply policy and charging to the data path remained constant. The PCN abstraction solves the problem of having to define policy and charging interfaces separately for each generation of core nodes, simplifying specification and implementation.
Historically, before the PCN concept was formalized, policy and charging specifications had to explicitly list all applicable nodes (SGSN, GGSN, etc.). This became cumbersome with each new network generation and the introduction of new node types like the TDF. The PCN term, introduced around Release 8 with the enhancement of the PCC architecture, provides a future-proof categorization. It allows the PCC framework (PCRF, PCEF) to be defined in terms of logical functions (e.g., 'the PCN shall enforce QoS') rather than specific physical entities.
This abstraction addresses the limitation of tightly coupling policy control with specific node implementations. It enables a more flexible, service-based architecture where the policy decision point (PCRF) can issue commands to any node that fulfills the PCN role, whether it's a legacy GGSN or a cloud-native P-GW. It also facilitates network sharing and multi-vendor interoperability by defining common behavioral requirements for all packet core gateways.
Key Features
- Abstracts specific packet core nodes (SGSN, GGSN, S-GW, P-GW, TDF) under a single functional category
- Serves as the Policy and Charging Enforcement Point (PCEF) for user plane traffic
- Generates Charging Data Records (CDRs) for offline billing
- Interacts with Online Charging System (OCS) for real-time credit control
- Enforces QoS policies (gating, bandwidth limiting, priority marking) as directed by PCRF
- Anchors user sessions and manages data bearers/tunnels (GTP)
Evolution Across Releases
The term PCN is formally defined within the Policy and Charging Control (PCC) architecture specifications. It consolidates references to packet-switched core nodes for policy enforcement and charging purposes. The initial architecture includes SGSN, GGSN, and the newly defined S-GW and P-GW from EPS, establishing them all as PCNs subject to common PCC procedures via interfaces like Gx and Gy.
Defining Specifications
| Specification | Title |
|---|---|
| TS 32.251 | 3GPP TR 32.251 |
| TS 32.298 | 3GPP TR 32.298 |